1. Field of the Invention
This invention relates to optical fibers and, more particularly, to rare-earth-doped silica optical fibers for high power optical amplifier applications.
2. Discussion of the Related Art
Because of their high performance and cost effectiveness, rare-earth-doped fiber amplifiers (REDFAs), especially erbium-doped fiber amplifiers (EDFAs), are widely used in silica fiber-optic communication systems such as, for example, long-haul transport and CATV applications. Innovative design and optimization of rare-earth-doped fibers (REDFs), especially erbium-doped fibers (EDFs), have both played a critical role in these applications. In particular, designs that confine the optical mode field and control the erbium distribution enable efficient, low-noise amplification of C-band and L-band light at low and medium optical power levels. On the other hand, for high power applications large mode area fiber with low numeric aperture (NA) lowers the pump intensity, thereby reducing deleterious nonlinear effects and pump excited state absorption, which are discussed below. See, for example, P. F. Wysocki et aL, “Erbium-doped fiber amplifiers: advanced topics,” Rare-Earth-Doped Fiber Lasers and Amplifiers, ed. M. J. Digonnet, p. 583 (2001). High performance EDFAs for high power WDM applications require EDFs with both large mode area and flat spectral shape. However, commercially available low NA EDF is typically not suitable for WDM applications because the gain spectrum is not sufficiently flat.
It is well known that EDF designs need to be separately optimized for each different amplifier application. Regardless of the design, however, high performance EDFs for WDM applications should possess certain fundamental characteristics: high power conversion efficiency at the requisite pump power level and relatively flat spectral gain shape over the wavelength range of operation; e.g., the range covered by the C-band (approximately 1525 nm to 1570 nm) and L-band (approximately 1570 nm to 1610 nm).
Consider, for example, conventional silica EDFs in which in the core is doped with aluminum (Al) and germanium (Ge) as well as erbium (Er). High Al concentration is necessary to achieve a relatively flat gain spectrum over the C-band and L-band. In addition, for high power applications, low NA is desirable for two reasons: first, it reduces deleterious nonlinear effects (e.g., four-wave mixing); and second, it reduces excited state absorption at 980 nm, thereby improving power conversion efficiency when using 980 nm pump light. Trying to achieve both of these characteristics in conventional EDFs presents the following conundrum: high Al concentration and low NA cannot be achieved simultaneously because Al increases the refractive index of the core, which increases the NA.
Thus, a need remains in the art for an REDF, especially an EDF, that has both a relatively flat gain spectrum and a low core NA, making the REDF suitable for high power REDFA applications.